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The SIM921 AC Resistance Bridge is a precision, low-noise instrument
designed for cryogenic thermometry applications. With excitation
power below 100 aW, thermistors and other resistive samples can
be measured at temperatures below 50 mK with negligible self-heating
errors.
Measuring Resistance
The SIM921 measures resistance using a four-wire circuit, eliminating
the direct effect of lead resistance on the result. Thermal EMFs
and amplifier offset drifts are avoided by using an AC excitation
current source.
The excitation frequency can be adjusted from 2 Hz to 60 Hz, either
from the front panel or over the computer interface. This flexibility
allows the user to operate at a synchronous sub-harmonic of the
power line frequency (such as 15 Hz/12.5 Hz) or at some incommensurate
frequency, depending on requirements. Multiple SIM921s can be operated
at different frequencies in the same experimental set-up without
risking in-band crosstalk. Also, for very high impedance measurements,
the variable frequency makes it possible to probe any capacitive
effects in the resistance result. Excitations are sinusoidal, eliminating
the high-frequency harmonic content associated with square wave
excitations.
The actual determination of resistance is achieved ratiometrically,
passing the selected excitation current through both an internal,
high-stability reference resistor as well as the user's device under
measurement. An internal autocal is available to calibrate the two
arms of the ratio readout for greater accuracy.
Excitation
Two excitation modes, constant current and constant voltage, are
available with the SIM921. Most low temperature thermometry applications
use negative temperature-coefficient resistors. The constant voltage
mode has the benefit of decreasing the power dissipated in the thermometer
as the temperature drops. In this mode, the SIM921 servos the applied
AC excitation to maintain the selected voltage across the user's
resistor.
Constant current mode is appropriate when measuring small resistances,
such as characterizing superconducting transitions. In constant
current mode, the internal reference resistor is used as the input
to the servo, giving a constant current equal to the selected voltage
divided by half the resistance range (for instance, 100 µV on the
20 kΩ range gives 10 nA rms excitation current).
Phase Sensitive Detection
A pair of dual-phase sinusoidal AC demodulators in the SIM921 provide
excellent signal-to-noise ratio in the most difficult test conditions.
Further, dual-phase demodulation enables resistance and phase-shift
measurements. Large phase shifts can warn the user of excessive
lead reactance. Selectable post-demodulation time constants from
300 ms to 300 s give you complete control over the trade-off between
measurement response time and ultimate resolution.
Autorange
When autoranging is selected, the SIM921 dynamically adjusts the
bridge amplifier gains for optimal performance with small signals
and rescales the display based on the measured result. By disabling
autoranging, the display range is held fixed, and the bridge amplifiers
are kept at full-scale gain. This can be particularly important
when using the SIM921 in a control loop application. In both autorange
and manual range mode, the excitation settings are never changed
by the instrument, ensuring the user complete control over measurement
conditions in an experiment.
Thermometry
The SIM921 is compatible with all resistive sensors including NTC
sensors (germanium, Carbon-Glass™,
carbon-composition, Cernox™,
ruthenium oxide, etc.), and PTC sensors (rhodium-iron RTD, platinum
RTD, etc.). Up to four user-calibration curves (ohm to kelvin),
with 200 points of data each, can be uploaded to the instrument
via the computer interface.
Output
In addition to the display output and computer interface, an analog
output provides a DC voltage proportional to either resistance or
temperature. The user has full control over the scale (V/K or V/Ω
) and offset (K or Ω ) of this output.
Temperature Control
The analog output signal is well suited to connect with the SIM960
Analog PID Controller. This combination of modules provides a flexible
and cost-effective temperature control solution.
Front-Panel Display
The primary readout is an easy-to-read 5½-digit LED display (statically
driven for low noise). This display can show measured value (resistance
or temperature), value minus offset, phase shift, offset, excitation
frequency, analog output scale, and cal-curve. Separate bar-style
displays indicate the resistance range, excitation, and output time
constant, as well as excitation mode (current or voltage) and autorange
setting.
Interfaces
All instrument parameters can be controlled and displayed on the
front panel or set and queried over the computer interface. The
analog DC output is available on a front-panel BNC connector.
The rear panel has a standard 9-pin D-sub connector for the sensor.
Power and serial communications are via the 15-pin D-sub connector
which mates with the SIM900 mainframe. Stand-alone operation of
the SIM921 is possible by providing ±15 V and +5 V power directly
on the 15-pin connector.
Resolution
Resolution is given in the table below. Upper values give excitation
current, while lower values are typical rms resistance noise measured
at 50 % full scale on a room temperature resistor with a 3 s output
time constant.
| |
Excitation
|
| Range |
30 mV |
10 mV |
3 mV |
1 mV |
300 µV |
100 µV |
30 µV |
10 µV |
3 µV |
| 20 mΩ |
N/A |
N/A |
N/A |
N/A |
N/A |
100 mA
44 µΩ |
3 mA
130 µΩ |
1 mA
510 µΩ |
300 µA
1.5 mΩ |
| 200 mΩ |
N/A |
N/A |
N/A |
10 mA
8.9 µΩ |
3 mA
12 µΩ |
1 mA
32 µΩ |
300 µA
120 µΩ |
100 µA
590 µΩ |
30 µA
1.4 mΩ |
| 2 Ω |
N/A |
10 mA
4.3 µΩ |
3mA
5.5 µΩ |
1 mA
7.9 µΩ |
300 µA
23 µΩ |
100 µA
70 µΩ |
30 µA
220 µΩ |
10 µA
730 µΩ |
3 µA
1.8 mΩ |
| 20 Ω |
3 mA
20 µΩ |
3 mA
21 µΩ |
300 µA
33 µΩ |
100 µA
41 µΩ |
30 µA
100 µΩ |
10 µA
390 µΩ |
3 µA
1.7 mΩ |
1 µA
4.1 mΩ |
300 nA
10 mΩ |
| 200 Ω |
300 µA
200 µΩ |
100 µA
200 µΩ |
30 µA
370 µΩ |
10 µA
430 µΩ |
3 µA
1.1 mΩ |
1 µA
2.8 mΩ |
300 nA
9.7 mΩ |
100 nA
25 mΩ |
30 nA
120 mΩ |
| 2 kΩ |
30 µA
2.0 mΩ |
10 µA
2.0 mΩ |
30 µA
2.9 mΩ |
1 µA
4.0 mΩ |
300 nA
12 mΩ |
100 nA
40 mΩ |
30 nA
120 mΩ |
10 nA
300 mΩ |
3 nA
900 mΩ |
| 20 kΩ |
3 µA
20 mΩ |
1 µA
25 mΩ |
300 nA
31 mΩ |
100 nA
56 mΩ |
30 nA
200 mΩ |
10 nA
640 mΩ |
3 nA
2.4 Ω |
1 nA
5.3 Ω |
300 pA
23 Ω |
| 200 kΩ |
300 nA
250 mΩ |
100 nA
350 mΩ |
30 nA
640 mΩ |
10 nA
1.4 Ω |
3 nA
4.5 Ω |
1 nA
16 Ω |
300 pA
47 Ω |
100 pA
150 Ω |
30 pA
710 Ω |
| 2 MΩ |
30 nA
3.4 Ω |
10 nA
5.9 Ω |
3 nA
16 Ω |
1 nA
46 Ω |
300 pA
190 Ω |
100 pA
480 Ω |
30 pA
1.7 kΩ |
10 pA
5.4 kΩ |
3 pA
15 kΩ |
| 20 MΩ |
3 nA
50 Ω |
1 nA
190 Ω |
300 pA
540 Ω |
100 pA
1.1 kΩ |
30 pA
5.4 kΩ |
10 pA
12 kΩ |
3 pA
56 kΩ |
1 pA
180 kΩ |
300 fA
750 kΩ |
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